The present invention relates to a method of transferring information between microcomputers in a decentralized process control system.
As described in the West German journal "Elektrisches Nachrichtenwesen", Vol. 52, No. 3, 1977, pp. 231-236, in process control systems, the processors and memories available until that time became more economical with increasing centralization; therefore, faster and larger central control units were used whose costs increased more slowly than their efficieny. However, the use of a central control unit for a large number of tasks requires that the tasks be handled by the processor successively in time. To this end, the tasks, parts of which occur in parallel, must be arranged in a time sequence ahead of the processor, and the results must be distributed to parallel receivers. This creates special problems, such as the operational organization for executing the tasks at the correct time, program interrupts by tasks of higher priority, and steps to avoid failures of the central processor. Further problems are caused by the high cost of the basic equipment of a center and the lack of flexibility when the system has to be adapted to a new configuration level. These problems show the limits of centralized data processing.
Tasks frequently occur not only sequentially but also in parallel, such as the processing of subscribers' wishes in telephone switching centers. Microcomputers make it possible to use stored-program-controlled processing at decentralized locations where the tasks occur. The microcomputers are interconnected to form a multiprocessing system. Each microcomputer performs a share of the control tasks of the overall system.
Such decentralized process control systems generally improve the throughput, flexibility, availability, and reliability of computers.
West German patent publication DE 32 12 556 discloses a switching system with distributed control. The data bus consists, for example, of eight wires for parallel data transfer and of two single-wire control buses. All signalling data required to set up, hold, and release a call is transferred over these buses. For the data transfer via a bus controller and the data bus, the latter can be granted on a fixed cyclical basis, without regard to priorities. However, bus mastership is granted centrally, so that difficulties will arise if the central arbitration fails.
West German patent publication DE 27 24 431 discloses a method of switching calls through a telecommunication switching system with decentralized control wherein each decentralized control continuously receives the states of all other decentralized controls which are important to the connection setup of the terminating equipment, and wherein, to establish the connections, the decentralized controls send control instructions to peripheral equipment or other decentralized controls which take the already stored current states of the remaining centralized controls into account. For the interaction of the controls, a multiwire signalling line, a separate control line, a multiwire connecting line, and a stepping line are employed. The signalling line is used by a decentralized control only as long as it is needed for the transfer from the sending data station or to the receiving data station. Over the separate control line, a request to read the message transferred over the signalling line is sent from the transmitting decentralized control to the decentralized controls which are not transmitting, and the receiving decentralized controls are informed that the processing of the transmitted messages has not been completed yet. The stepping line interconnects the decentralized controls in the form of a ring circuit, and transfers signals such that the following decentralized control is prepared for the transmit mode as soon as the transmission of the preceding decentralized control on the signalling line has been completed. A central counter is provided which is cyclically incremented after the absence of a signal on the separate control line or the signalling line for a predetermined safety time, with each number of the possible counts assigned a particular decentralized control, and with the count output connected via a decoder and a connecting line to the individual data stations and successively switching the latter to the transmit state.
Here, too, a central element, the counter, is present, which poses special reliability problems. In addition, many wires are needed to transfer the information between the decentralized controls.
West German patent publication DE 27 08 244 discloses a method of controlling a switching system with a group of small computers each of which is connected to a group of independent microcomputers and to a master computer for priority control.
The small computers are interconnected by a bus over which information is transferred to buffer memories associated with the small computers. These buffer memories serve to match the signal flows between the computers and the bus because the devices connected to the bus can receive or deliver the signals only at precisely specified instants. In the case of a time-division-multiplex telephone exchange, a frame with 32 channels is used for the information transfer over the bus. In one embodiment, two channels are reserved for each computer of a total of 16 computers, one channel containing the address of the small computer, and the other containing the actual information. An address comparator for each small computer senses the information intended for the associated small computer. Sometimes, however, this systematic reservation of a transmission time for each small computer is not adapted to the traffic conditions. It is proposed, therefore, to assign the informationtransmission times with the aid of the master computer.
This TDM method uses a central clock generator, so that it suffers from the disadvantages described above. For smaller process control systems, time-division multiplexing is also too costly.
A switching system with central control and decentralized controls is disclosed in West German patent publication DE 27 20 833. There, the decentralized controls are interconnected by an address bus in addition to the actual data bus, so that a large number of control lines is present again. In addition, a relatively large amount of hardware and software is required to control the information transfer, particularly for bus control, so that it is doubtful whether economic use is possible, especially in smaller systems.